1. Field of the Invention
The present invention relates to a brake fluid pressure retaining unit that applies brake fluid pressure to the inside of a wheel cylinder even after the brake is released and thus maintains a predetermined brake fluid pressure so as to prevent a vehicle from moving backward (creeping down), for example, even on a steep hill.
2. Description of the Related Art
For instance, to start again a vehicle in a state of being temporarily stopped on an uphill slope such as an uphill road, a driver of a vehicle with a manual transmission (manual transmission vehicle) needs to perform troublesome operation, specifically, to cause partial clutch engagement while maneuvering a parking brake and depress an accelerator pedal to start the vehicle.
On the other hand, a vehicle with an automatic transmission (automatic transmission vehicle) is capable of providing creep force. However, for demands for improvement in fuel economy, environmental friendliness, and the like, it is required for vehicles to stop in an idle stop state or to reduce the engine idle speed. For this reason, even an automatic transmission vehicle moves backward when a driver changes pedals to depress from a brake pedal to an accelerator pedal on an uphill road, because creep force is lost or reduced.
Therefore, to prevent a vehicle from moving backward when starting the vehicle on a hill, a brake fluid pressure retaining unit has been proposed which continues to apply brake fluid pressure to the inside of a wheel cylinder even after a driver has released the depression of a brake pedal (for example, see Patent Document 1).
In this brake fluid pressure retaining unit, a brake fluid pressure circuit connecting a wheel cylinder and a master cylinder for generating brake fluid pressure according to depression force on a brake pedal caused by a driver, is provided with an electromagnetic valve capable of freely opening/closing the brake fluid pressure circuit, a relief valve for abruptly reducing the brake fluid pressure to a level where creep force is generated, and an orifice for making a decreasing rate for the brake fluid pressure inside the wheel cylinder smaller than a decreasing rate for the depression force on a brake pedal caused by a driver.
In this brake fluid pressure retaining unit, an extremely high pressure is applied to the wheel cylinder because a driver depresses a brake pedal with greater force than needed to obtain braking force. However, by use of the relief valve, the high brake fluid pressure can be abruptly reduce to a pressure that produces creep force, and further, the brake fluid pressure is gradually reduced by use of the orifice. Accordingly, when starting the vehicle on a hill, brake fluid pressure is continuously applied to the inside of the wheel cylinder after the driver releases the depression of the brake pedal until the driver starts the vehicle, thus preventing the vehicle from moving backward when it is started.
(Patent Document 1)
Japanese Patent Laid-Open No. 2000-190828 (pp 7–8, FIGS. 1 and 3)
However, in the above-described brake fluid pressure retaining unit, since a relief pressure obtained by use of the relief valve is constant, if, on a steep hill, braking force attributable to the relief pressure (brake fluid pressure) becomes less than creeping-down force due to a hill (force due to the weight of the vehicle), it will results that the vehicle moves backward. Moreover, in this brake fluid pressure retaining unit, the relief valve and the orifice are needed in addition to the electromagnetic valve. Further, pressure reduction characteristics at the time of slow release (i.e., when fluid pressure is slowly reduced) depend on the orifice (throttle) and are constant. Furthermore, the orifice is very difficult to manage during a manufacturing process and also expensive.
In this brake fluid pressure retaining unit in particular, since a spring provided in the relief valve produces a constant spring force and the diameter of the orifice is constant, only a constant relief pressure can be produced even when the gradient of a hill changes. Therefore, a need to quickly reduce the fluid pressure depending on the angle of inclination cannot be met. For instance, on a downhill slope, a driver wants to quickly reduce the brake fluid pressure but cannot do so actually, and as a result, drag occurs.
Moreover, it is also conceivable that the relief pressure is variably controlled by use of a solenoid valve. However, according to a conventional and general solenoid control method, the voltage value is duty cycle controlled periodically, and accordingly, the following problems arise. As shown in FIG. 9, when the voltage value is duty cycle controlled and the current value is controlled to be constant, it is difficult to maintain a constant oil pressure. That is, when the current value is intended to be constant, as can be seen from FIG. 9, the current value may become both a value that causes an electromagnetic valve to open and a value that causes the electromagnetic valve to close, and accordingly, the solenoid valve will be either in an open state or closed state depending on the state of the electromagnetic valve.
That is to say, when the current value is made constant based on the state of the electromagnetic valve before controlled, the maintaining oil pressure is greatly affected by this fact, and a balance is caught at a low value of maintaining oil pressure that brings the electromagnetic valve in a closed state. In the case of the balance at this value of maintaining oil pressure, when the electromagnetic valve is desired to be switched from a closed state to an open state, and again to a closed state (i.e., when the maintaining oil pressure is desired to be slightly lowered), the maintaining oil pressure is in some cases excessively reduced more than needed to make the electromagnetic, which has been once brought in an open state, return to a closed state. Such a problem is caused by the fact that the current value is controlled by performing periodical duty cycle control of the voltage value, and accordingly, instantaneous response ability of the current value is affected by the self-heating of the electromagnetic valve and the like.